Is work the result of taking into consideration one vector and the object's movement along that vector then (force and distance)..? And net work is taking into account what happens at the end o_o..? But why is net work equal to change in kinetic energy?

Also, what average force?

Suppose in a frictionless environment you perform 20 N*m of work northeast, then 40 N*m of work southwest, then 20 N*m of work northeast, and you're back where you started. (Note that this means the object started at rest, accelerated for a while, slowed back down and started going the other way, then slowed back down again.) From here, there are two different interpretations of the sum of that work.

Net work is just looking at the initial state and the final state and observing the changes. This is the vector sum of the work; in this case, 0 N*m. This measures the change in the overall state of the system.

Total work is the path integral of power (that is, the dot product of the force vector and the velocity vector, which is a scalar) over the trajectory of the object. You can think of it as chopping up the force over infinitely many infinitely small segments of the distance and adding up the scalar magnitudes of those pieces. That gives you 80 N*m. This measures the effort expended by whatever it is doing the work.

If you take that same chopped-up description of the forces that were acting on the object and average them all out, then you could call that the average force. If you started with a system in the same initial state, and you applied this average force vector to the object for the same duration of time, then you would end up with a system in the same end state.

This is actually WHY net work is equal to change in kinetic energy. This description represents the simplest possible way to transform the initial state into the final state. If you were to add any other forces to this description, you would have to add balanced opposing forces to push it back, or else you wouldn't end up in the same end state. And since those forces have to be balanced, they can't possibly cause a change in kinetic energy. That means the only change in kinetic energy that actually matters in the end is that average force over that final displacement.

(Footnote: The sum of infinite pieces thing is called an integral. It's the continuous expansion of the idea of a sum of discrete quantities. It's one of the two basic operations of calculus.)

I don't know what dot product is, but it does make sense now! Thank-you!

The dot product of two vectors is easy. Given vectors [a_1, b_1, ...] and [a_2, b_2, ...] of the same length, the dot product is:

(a_1 * b_1) + (a_2 * b_2) + ...

Surprisingly equivalently, the dot product is ALSO the product of the vector's lengths multiplied by the cosine of the angle between them, that is, ||v_1|| * ||v_2|| * cos(θ).

Don't ask for a graphical interpretation of what this actually represents geometrically, because it doesn't have a strict meaning. And my theoretical geometry skill isn't good enough to have an intuition on when I would want to use the dot product of two vectors outside of formulas that other people have already derived.

That said, it has a lot of useful properties. For example, if the dot product of two vectors equals 0, then the vectors are perpendicular, and if two vectors are parallel, then their dot product is the product of their lengths. (The dot product of a vector with itself is therefore the square of its length, which demonstrates the Pythagorean Theorem.)

Sadly, I've never learned dot product, so I'm not sure what the square brackets mean D:, but it does look useful.


I'm reviewing a question that essentially tells me about a cannon firing a cannonball, and that before it fires, the momentum of both the cannon and the ball together is 0. Therefore, after it fires, the momentum of the cannon and the ball should make 0 again..but I can't seem to remember a logical reason for why that is.

EDIT: Random question - is it a more useful habit to label things like the cannon and cannonball m_1 and m_2 or m_cannon and m_ball ?

EDIT2: struck out question about momentum before and after. I suppose it's basically the same as momentum is conserved, therefore, if two things are at rest and suddenly move because of each other, momentum is still conserved therefore their total momentum added together is 0?

The square brackets are just an element-wise description of a vector. [1, 1] represents a two-dimensional vector with magnitude sqrt(2) pointing in a direction 45 degrees above the x axis -- that is, [1, 1] describes a vector starting at (0, 0) and going to (1, 1). (It also describes a vector starting at (1, 2) and going to (2, 3) -- the magnitude and direction of a vector is independent of its position in space.)

Labeling your variables descriptively is always good. :P I'd probably use m_c and m_b instead of spelling out "cannon" and "ball", but you should only use subscripted numbers when you're actually referring to numeric values, such as a position in a list or a point in time (which is why x_0 for "initial position" is good).

Your EDIT2 is exactly correct.

That way of writing down vectors seems really useful :o. It's like the vector don't need to be drawn anymore and their actual location doesn't really matter anymore..?


There was a multiple choice question on my test saying something like: "Why don't we account for the internal forces of an object when calculating its momentum?" And then there were four things to select. I guessed a choice that said internal forces were somehow "conserved" too, but frankly I have no idea why '~'

Well, you can't even DRAW vectors of more than two dimensions. :P

Sometimes you'll see it without commas, just spaces, and technically you're supposed to write them vertically:
https://wikimedia.org/api/rest_v1/me...376499cb2241ec

Newton's first law. An object in motion stays in motion unless an OUTSIDE force acts upon it. Internal forces can only make the parts move relative to each other, but that can't change the center of mass of the system.

This could also be argued using Newton's third law. Any internal force must necessarily have an equal and opposite internal force, so outside the system there's no net force.

EDIT: However, the question gets MUCH more interesting if you want to take relativity into account. Since momentum is proportional to energy, you might want to ask if the energy from internal forces can influence momentum.

The answer to that is: We don't know! So far we haven't seen any measurable violations of it. But there are theories that suggest that there might be an exotic force we haven't discovered that could increase an object's relativistic momentum without an outside force acting on it. Such a force could possibly be an explanation for dark matter (not actually matter, but this exotic force creating momentum that can't be explained by mass in motion) if it existed, but its influence would be so subtle at human scales that we'd have a very difficult time measuring it -- it's already stupidly hard to measure GRAVITY at human scales.

My son's in 4th grade, so I've been seeing a lot of Common Core stuff over the last few years.

I used to really hate Common Core.

Then things started to click. Turns out the problem isn't with Common Core in and of itself. The problem is that no one knows how to teach it correctly.

This video (https://www.youtube.com/watch?v=aLCO6pG6JDY) is a fairly accurate description of the counting-up method of subtraction that's taught as part of Common Core. It seems ridiculous, doesn't it?

Well, it IS ridiculous, when you do it like THAT.

The problem isn't with the technique, though. Believe it or not, the technique is actually used MORE OFTEN than traditional subtraction with borrowing in the real world.

"I don't believe you, Coda!"

No? Well, check this out:

"39, 40, 50, 60, 70, 80, 90, a buck, two bucks, three bucks, three twenty-five. That's two pennies, six dimes, two dollars, and a quarter; your change is $2.87, have a nice day!"

Why don't they teach it that way in schools? >_< Parents could actually help their kids with their homework!

Every step of that counting-up process is a trivial thing to do in your head. It's all small, round numbers until the very last step -- 2 + 60 + 200 is trivial!

In my opinion, ideally, they teach basic subtraction and that common core sort of way for mental math. Like "here's how to do it fast on paper" then, "here's a way to think about it in life but not on paper." I was taught normal subtraction and..that common core technique reminds me of learning mental math (but I wasn't good at that and we didn't focus on it too much so I don't really remember too well).
I don't really know what common core is though. I think I was in 5th or 6th grade when it was first implemented (that's 2010 I think). And then in 7th grade I went to a school that could care less about state/national standards.



Momentum is proportional to energy..? Is it related to change in momentum over time equals change in KE over distance (I remembered this as a part of some problem on my test..is it related?)? Oh, is that say, we don't know if an object of a higher temperature influences momentum? Err, I hear about dark matter on occasion but what is that?

Something about Newton's 3rd law was shown on the test--but I didn't remember what law no. 3 was. I can understand that explanation but, using Newton's first law, I'm not sure how it's different from the 3rd law..? Internal forces don't count as an outside force and the internal stuff technically moves but overall the object doesn't move anyway and so it doesn't influence the momentum?

I definitely agree that they need to teach multiple techniques. The INTENT of the curriculum is to equip kids with tools to be able to learn in an efficient way, and that's not well-served by trading out a technique that doesn't work for some kids for a technique that doesn't work for other kids.

Really, it's not so much that they're different ways of answering the question. Rather, the third law is WHY the first law holds.

EDIT: Screw it. Hold on for the other question. I'm going to go change stuff in the admin CP.

If common core is replacing "old-school" subtraction with that though, uh, future math classes are going to be tough.

Ohh, thanks that makes sense now.

Admin control panel..?

If my son's classes are any indication, they're not completely replacing it. He still knows how to do subtraction with borrowing.

Yes, Trisphee's admin control panel. You'll see.

As for momentum and energy... that goes back to E² = p²c² + (mc²)². This is saying that momentum and mass are related in an interchangeable way.

But what if there WERE something else that contributed to that total energy figure? Some internal force inside the object? Then you'd have to add another term to the equation.

You suggested temperature. Well, that one we actually DO know the answer to -- it doesn't, because temperature measures the average kinetic energy of the particles inside the object relative to each other. The same argument concerning Newton's first and third laws applies here -- any motion inside the system would have to be countered by an opposite motion. If it WASN'T countered by an opposite motion, then that would be contributing to the momentum of the system, which is contrary to the definition of temperature measuring motion internal to the system.

BUT!

Given that the object isn't exploding, there must be some force holding it together. It could be atomic bonds, it could be gravity, it could be magnetism, whatever. So does this force contribute energy that's equivalent to momentum and mass?

That's what we don't know. We can't prove it doesn't, but if it does then it's such a small contribution that we haven't detected it yet.

But there could be some other force that we don't know about with a very weak effect that operates over very large scales, so an Earth-based experiment would have a very hard time doing anything with it. If there is, then this might be an explanation for why the universe isn't expanding faster than it is, and finding a way to measure how much of it there is could give us a clue as to the far-future fate of the universe.

Now that I think about it, I didn't really learn math basics from school--school gave me problems and my parents taught me how subtraction and fractions and things worked. I still remember asking teachers what in the world I was supposed to be doing, then giving them blank stares after they responded.

Oh woops, temperature would just make things go faster but they still cancel each other out.
That's really interesting! And kind of mind boggling. If an object were disassembled into bits of protons, neutrons, and electrons, and whatever parts there are, I wonder if they'd have the same mass XD.

Nope! I mentioned this before: The energy absorbed or released by chemical and nuclear reactions does in fact influence the system's rest mass! And we can use E=mc² to describe just how much.

One example: A hydrogen nucleus (1 proton, 1 neutron) has a mass of 2.01410178 amu. A helium nucleus (2 protons, 2 neutrons) has a mass of 4.002602 amu. But that means two hydrogen nuclei (2 protons, 2 neutrons) have a combined mass of 4.02820356 amu. What's the ~0.0256 amu difference? It's the binding energy in the nucleus holding the protons together!

0.0256 amu is roughly 4.25x10^-29 kg.

E = mc[sup]2[/i]
E = (4.25x10^-29 kg)(3.00x10⁸ m/s)²
E = 3.82x10^-12 J

And so we predict that this would be the energy released by two hydrogen nuclei becoming a helium nucleus in a fusion reaction. It seems like a pretty small number, but then you can go calculate that red light (remember, the LEAST energetic visible light) has a kinetic energy of only 2.84x10[sup]-19[sup] J and you see just how powerful that actually is.

It should be noted that sometimes bond energy can have a negative contribution to mass -- this is usually unstable. Further discussion of the concept starts getting into some chemistry stuff, so I'll defer that.

"But Coda, didn't you just say a couple posts ago that the forces binding objects together DOESN'T have an effect?"

I did indeed. I'll confess that I wasn't being wholly precise before, since this isn't a masters-level physics course here.

The difference is that atomic/molecular binding energy is part of the rest mass. It acts just like any other mass: it has inertia and it participates in gravity. You can measure it just fine using either of those principles, and in fact the only way to measure that as distinct from the "base" mass (if such a thing existed) would be to break all of the bonds and measure the difference.

This is so fundamental that it turns out that around 99% of the mass of a proton is the binding energy holding its constituent quarks together! Only around 1% is the rest mass of the three quarks inside (inasmuch as you can say that quarks have rest mass, because they're unstable in isolation).

So really, what I was saying that we don't know is if there's a contribution to the total E of a system that you can't measure by observing its rest mass or its momentum.

Woah! So energy is mass..? Or most of mass is energy? Doesn't that mean mass is literally energy bunched up together O_o? And that what's happening at nuclear power plants is more or less spreading that locked and bunched together energy out?

Ohh woops. When energy is released mass is lost, and taking the stuff apart means releasing all the energy. But then again protons and electrons aren't even masses either (I forgot quarks existed, well I never knew what they were until now ^^;;)...What even is mass, it seems that literally everything might as well be internal forces o_o.

Oh! But why do quarks count as rest mass? Aren't they energy as well x'D.

The internal forces aren't accounted for is because the internal forces are part of the rest mass..? And they, because of Newton's third law, aren't going to suddenly change the object's rest mass?

That's exactly what E = mc² means!

You could look at it that way!

I think the bulb may have just lit up. :D

I'll point out that particle physicists measure mass in electron-volts (1 eV = 1.6x10^-19 J) instead of some unit derived from the gram.

They count as rest mass from the perspective of separating binding energy from everything else, but you're right: that rest mass itself is made of energy! That energy has a charge and a spin and other properties, and it's got the same wave/particle duality that photons have, but it's turtles all the way down.

Strictly speaking, it's not the forces themselves, it's the energy resulting from the forces interacting. But basically, yes.

I say basically because radioactivity violates the simplistic reading. The mass has changed, but no outside force has acted upon an atom to make it give off alpha or beta particles; it's one of the things that currently appear to be truly spontaneous in physics. But the system taken as a whole, if you look at everything as energy equivalence instead of distinguishing between mass and energy, obeys it -- adding up the nucleus, the photon of energy, and the emitted particle means the final system has the same mass-energy as the initial one.

I forgot that radioactivity existed :o. Granted, all I know about it is that carbon dating is used and that's related. And carbon dating has to do with emitting something. And I guess emitting something means losing mass or energy.

Err, so anything internal is not accounted for because it's all part of the rest mass. And for things like atoms bouncing around, Newton's third law says that within the object, that sort of bouncing will be balanced out by another atom because within the object itself there is an equal and opposite reaction..?


Electron volts! I don't know what a volt is but it's interesting they use energy, and that it's electrons. Why not quarks xD?

Random tidbit! When dealing with petty cash, a quarter, a dime, and a nickel equal 40 cents! Seems obvious, but it's amazing how useful this little bit of addition is, and how it's not always obvious when you first start out as a cashier.

Electron-volts are like light-years. You're not measuring electrons or volts any more than you're measuring light or time; it's a measure of energy based on an electron moving across one volt, analogous to deriving a unit of distance from how far light travels in a year.

"Why not quarks" is because you can't measure quarks directly, because they don't occur in isolation; they ALWAYS occur in bound groups and will instantly transform into something else if you manage to break them apart. Electrons are, as far as we can tell, actually fundamental, instead of being made up of other things.

And yes, your conclusion about rest mass is accurate as far as modern science can tell.

Ohh that's interesting. I've been (and still am) confused by the idea of stuff multiplying together to make something. The light example makes it clearer.

Oh woops, electrons are very different from protons and neutrons. Err, but what about protons and neutrons..? Do they matter? Are they part of the "volt"? Not that I know what's going on with the electron volt though :x



Wow, that really is useful when I think about it. I'm not a cashier but the first thing I'd think of for 40 cents would be 4 dimes, which I suppose is a waste of dimes XD.

The only relevant part of the electron here is that it's a single unit of negative electrical charge. Everything else about it doesn't matter for this definition.

A volt... hmm. I guess you could describe a volt as the electrical equivalent of the potential energy stored in an object suspended above the ground. It takes work to pick the object up against gravity; it takes work to pull a negative charge away from a positive one (and the electron-volt measures this amount of work). In either case, you can turn that potential back into kinetic energy by allowing the particle to move freely.

Err why does it matter that it's a single unit of negative electrical charge? Because that way it can be measured easily..? One electron is one unit?

So..a volt is how much stored up mechanical energy there is in an object (I looked at the units just now as ‎kg·m²·s⁻³·A⁻¹)?
Why potential? Why not kinetic? Is it because the particle is not moving freely within the object, but when it gets measured it is moving? (All I know is batteries and potatoes can have "voltage" measured with wires and multimeters, and the kg·m² makes me feel like it has to do with KE, since KE is kg. m2/s2).

One electron is one unit is exactly it -- it's as fundamental of a measurement as you can get, nothing arbitrary about it.

Stored-up electrical energy, not mechanical energy. It's possible to CONVERT between the two; that phenomenon is called "magnetism."

It's potential because it's not moving -- it CAN move, if it's allowed to. Just like a ball sitting on a table isn't moving, but it has potential energy: if you pull the table out from under it, that energy turns into kinetic energy.

A potato battery has measurable voltage because the metals you put in it have different electric potentials -- that is, given an opportunity, the electrons will move from one piece of metal to the other. And connecting a wire (or a multimeter!) between those pieces of metal provides that opportunity. It's like falling off a table; the electrons would much rather be on the lower-potential piece of metal than the higher-potential one.

Without going into too much detail right now because I'm tired... Charging a rechargeable battery is like rolling a boulder up a hill -- you put energy in, and later on, that energy can come back out.

So..electrical energy means energy from electrons?

I've not really learned about energy other than mechanical though--so the first thing I wonder at is that electrons are moving. Though when I think back I'm guessing electrical energy does not involve contact, whereas mechanical does, so that's why they're different? Though I've not yet learned about field forces other than the fact that gravity is an example of a field force.

Actually, what does moving mean? Don't electrons move in clouds or something all the time? Does moving mean it needs to transfer to another place..?

If I stick a piece of metal into a potato will it heat up? Or does it still need another end connected to the potato?

But the energy needs to be converted into chemical(?) energy? I sort of learned about rechargeable batteries last year but forgot a lot of it. Something about electrons moving places is all I remember. It's sort of interesting how energy needs(?)/can be converted to be stored and reused. I don't understand how solar panels and windmills and coal can possibly be used as energy when I think about it.

Energy OF electrons, not energy FROM electrons. But otherwise, close enough.

You are 100% correct that electrons are moving. Electricity on its own, however, DOES require contact, at least at the macro scale. That's what wires are for -- the electrons in the metal atoms are able to move without a lot of resistance (sort of analogous to friction), so when you pull electrons out of one side of a wire, electrons will ripple through to fill up the void you created. If this were to leave a void on the other side, though, the electrons would just flow back. (Yes, this means that electrons are constantly swirling around in metal, but if you think about it, you already knew that electrons are constantly swirling around -- it just means they're able to swirl around in a little bit bigger of a range than you might have expected.) In order to get an actual electrical current, then, you need to have a source of electrons on the other end to refill what you took out.

But electricity isn't the only effect in play here.

When electrons move, they create a magnetic field. And when magnetic fields move, they cause electrons to move. This is how windmills generate electricity, actually: the rotation moves a bunch of magnets along a bunch of coils of wire, so the magnets sort of drag electrons along, pushing some electrons down the wire and pulling some up from the other end. If you put a load between the far ends of the wire, like a light bulb, then you can make those moving electrons do work.

There are also electric fields. Protons have a charge of +1. Electrons have a charge of -1. A normal atom has equal numbers of protons and neutrons, so the net charge of that atom is 0, and so the net electric field of that atom is zero. But if an atom has more or fewer electrons, then it gets a net charge. We call such a charged particle an "ion" and it has a negative or positive charge, and it has an electric field with a strength and polarity based on that charge. (Ions can also be formed by multiple atoms bonded together; bonded atoms "share" electrons in a sense, so if any of those atoms are missing electrons or have excess electrons, the whole bonded group acts as an ion.) Oppositely-charged ions attract each other; ions with the same charge repel each other.

In clouds, you have ice crystals banging around a lot, and sometimes when they do, an electron will move over from one to another. This gives one crystal a positive charge, and one crystal a negative charge, and usually (for reasons that aren't well-understood) the bigger crystal is the one that ends up with a negative charge and the smaller one ends up with a positive charge. Those crystals do attract each other, but the attraction is fairly weak, and they have so much kinetic energy from being blown around in the air that they get separated. The heavier negative crystals filter towards the bottom of the cloud and the lighter positive ones filter towards the top. This creates a charge differential -- that is, an electrical potential, a place where an excess of electrons desperately wants to move over to a place with a shortage of electrons. Cloud lightning happens when the potential gets so great that it overcomes the resistance of the air (just because it's very high doesn't mean it's infinite) and the electrons take the shortest path from negative to positive.

Lightning strikes happen because that massive negative buildup in the cloud attracts positive charge in the earth to amass at the surface below. It takes a LOT of potential to overcome THAT much air, but as soon as even two little points get enough charge to get past the resistance of the shortest route between them, BOOM.

As for a potato battery: No, one piece of metal isn't enough. It requires two pieces of different kinds of metal (usually copper and zinc). There's phosphoric acid in the potato, and the speed that the acid can cause chemical reactions is entirely dependent on how fast the it can ionize the metal atoms, which is why you need two DIFFERENT kinds of metals -- one kind of metal will part with its electrons more readily than the other, and so that chemical reaction can happen more easily. This creates a charge differential, and if you connect a wire to the two kinds of metals, electrons can flow through it from the slower side to the faster side, facilitating that reaction.

As for why it doesn't pull the electrons back, the zinc ions react with the acid, creating zinc phosphate and positively-charged hydrogen atoms, which are drawn towards the now-negatively-charged copper. They pick up those extra electrons to become hydrogen atoms (and then react with other hydrogen atoms to create molecular hydrogen gas, which you can see bubbling at the copper metal if you set it up in a beaker instead of inside a potato). And so Newton's laws are conserved and the net charge of the system as a whole remains balanced, but you've derived work from the energy stored in chemical bonds in the potato.

Eventually, the potato will run out of phosphoric acid around the zinc metal, because it'll have all turned into zinc phosphate... and so the battery will be dead -- no more chemical reaction means no more ions means no more electron flow.

Some chemical reactions, such as the ones used in lithium batteries, create compounds that can be broken back apart easily by shoving a little bit of energy at them. And then if there's no path for electrons to flow (that is, you disconnect the load), then that reaction will sit there in tension, waiting for a source of electrons to let it finish.

Solar panels basically work by having a special kind of metal surface, so when a photon hits it, it knocks an electron off, so an electron gets pulled in to fill the void, and that creates a current flow.

Coal is a completely different thing. You can't actually create ELECTRICAL energy from coal. Instead, you burn coal, adding a little bit of energy to break apart a big molecule, and breaking the molecule apart releases more energy than it took to break it, so that creates heat. The heat is used to boil water, and the steam is used to spin a turbine, and the turbine is used just like the windmill above. Why yes, burning stuff IS an inefficient way of using energy.

Nuclear energy works the same way, except instead of breaking apart big molecules by setting them on fire, you have big atoms that flake off pieces of themselves on their own (radioactivity), and when they do that the particles they release have a chance of hitting other big atoms and making them split (atomic fission). In both cases, that produces energy... which gets used to boil water to spin a turbine to spin a magnet through a coil of wire. Nuclear power plants are actually not especially efficient either, but nuclear reactions generate SO MUCH raw energy that it doesn't much MATTER.

EDIT: O.o 55 aurum megapost

EDIT 2: I was wrong about the specific details of the chemistry of a potato battery. I've corrected that.

Lol xD. That's a long post.
Thanks for the detailed answer though!

I guess I should probably learn more about fields and currents before understanding how magnets generate electricity :/. Whenever I look for it, there's a lot about how generators are made up of different parts, and depending on the parts currents flow either back and forth or forwards, but I don't really understand exactly how the currents are flowing in the first place--as in why the magnets are causing that sort of current flow, and then there's how the weird circle part (I forgot the name) can make the electrons not flow back and forth but rather just flow forward.

Earth has a positive charge :o? Or it just has a more positive charge than the cloud..?

Soo... if a cloud is really high up, the shortest path might be between clouds or within the same cloud, and if it's lower on the ground electrons move from the bottom of the cloud to the ground?

If I had copper and zinc would it be extremely bad idea to connect them together and stick the ends into a power outlet xD?

Ohh, woops, I'd forgotten that chemical reactions were important in batteries.
Actually, because I forgot about it, a lot of it flew over my head. Though I do recall something about how one end needs to lose electrons and starts falling apart and the other end gains electrons and gets a coating.

What does "knocks an electron off" mean? It's not that the electron literally flies out of the metal surface is it..?

Earth has a net neutral electric charge. But you can induce a charge in an object by bringing another charged object near it; it'll push matching charges away and pull opposing charges near -- electrons are very, very mobile little things and it really doesn't take a whole lot to move them around. So there's a positively charged region that's formed below the cloud.

You're completely correct about the motion of lightning. Note, of course, that it's (ionized) air itself that ends up conducting the electricity, so the path of least resistance might not be the shortest STRAIGHT-LINE path.

If you had copper and zinc, it would be a bad idea to stick it into an outlet, but not because it would form a battery... it would be a bad idea because you would be creating a path of least resistance between the two sides of the power outlet and that much energy flowing through a couple tiny bits of metal will start a fire. The fact that they're different metals is irrelevant there.

You're remembering the chemistry relatively well there. All consistent!

And yes, the electron literally does get displaced from where it was. It sticks mostly close to the surface, skidding across to other atoms, pulled towards positively-charged things. It doesn't go flying off the surface -- that would be a beta ray, and that takes a lot more energy.

Oh woops. The reason for copper and zinc was for the chemical reaction(?) so all that's needed is anything with least resistance to stick into the power outlet? Oh, but does anything happen if only one side of a power outlet gets a metal strip stuck into it?

Depends on what the other end is connected to. If the other end is connected to YOU, and you're touching the ground with your bare skin, then the electricity will happily go from you to the ground, because the Earth itself can soak a ton of current.

Not wise.

Too bad there isn't a good way to see it without killing myself x'D.

Is electrons rushing through a human body ok? What's the alternative? I'm reminded of how people may survive or may not survive lightning strikes, but I don't remember what it was that let people live, or that killed people.

Electrons rush through your body all the time. It's how your nervous system works. And small amounts of extra current aren't damaging -- there's an experiment you can do where you set up a little buzzer circuit that makes a noise when electricity goes through it, and the pitch of the noise varies according to how much resistance there is between a couple of electrodes, and you can hold the electrodes in your fingers and control the sound just by how tightly you press your skin against the metal. It's harmless fun.

The problem is that resistance is a lot like friction. What happens to the energy that's lost due to friction in a kinetic system? It turns into heat. The same is true of electricity. We use this phenomenon intentionally in electric heaters and incandescent light bulbs (as we discussed in the past, get it hot enough and the light becomes visible). But your body has electrical resistance too, and if you put too much current through it, that heat will end up cooking you.

If you don't want to do this to your OWN body, of course, it's completely plausible to do this with an experimental setup. If you connect a power outlet to a couple of wires, and put those wires in opposite ends of a pickle, it'll light up like a fluorescent bulb! Don't actually TRY this at home unless you know what you're doing because you don't want to start fires or route the current through yourself -- after all, YOU don't want to be a light bulb.

The other risk is that your body DOES use electricity to communicate signals through your body. If you push too much current through those pathways, your nerves will get overloaded and all sorts of nasty stuff can happen -- for example, you could destabilize the electrical oscillator responsible for keeping your heart pumping, which will put you in cardiac arrest. (If you've ever heard of someone having to have a pacemaker implanted, it's an artificial replacement for that.)

Lightning strikes have a fairly good prognosis, medically -- roughly 80% of strike victims survive, although most have long-term injuries. It's easier for the electricity to flow over your skin to the ground than to pass through your body, and lightning is a single burst of power instead of sustained high-voltage exposure like you might get from touching electrical wires. Strike victims usually have to deal with problems like burns (from the electricity damaging the cells directly, not from the heat itself, which doesn't last long enough to cause burns that way), flash blindness, deafness, heart attacks, and seizures, but while those can be dangerous conditions, they're survivable with proper care.

It's a good thing youtube exists for seeing pickles light up xD. I wonder why it is that it seems like pickles light up at one end, then for the rest of the time only light up at the other end.
I'm guessing it's pickles because electricity runs through salt easier?

Thanks for all the info on the human body and lightning!
So..lightning strikes more often flow over the skin and maybe cause some burns in the process. Or lightning strikes might go through the body while causing burns and messing up how the nervous system(?) is communicating(?) using electrons(?) through synapses(?)(I don't really remember anything about synapses but it reminds me of electric things '~').

Yes, salt water is the reason. I think the reason it lights up at one side is because that's the side the electricity is coming in from, and the other side is where the now-much-less-energetic electrons go out. Not 100% sure.

Through nerves, not through synapses. Synapses are chemistry-based bridges between neurons, using ions to create a potential difference (that is, a voltage).

Ohh so that's what synapses were, woops.

But why would it matter that that's the side that electricity goes out because it's flowing either way?

Imagine pouring water through a funnel into a pipe. On one end, you've got lots of water splashing around and if you try to pour too much in it overflows. On the other end, the water comes out in a nice even stream. The pickle is an overflowing funnel -- the light happens because there's more energy coming in than can flow smoothly through the medium.

Ohh okay, that makes sense! I'm wondering why electrons give off light when they're not flowing smoothly, but I guess I might as well question why things make sounds when they hit other things xD

Electrical resistance is very much like friction, as I mentioned before. It converts kinetic energy into heat.

As for objects knocking into each other making noise: it's a shockwave! The speed of sound in an object is actually measuring how fast force can propagate through that material.

You already know Newton's third law. Object hits other object; other object exerts force back on first object. Well, imagine that down at the atomic level. The atoms at the point of collision get knocked into the atoms behind them, which get knocked into the atoms behind them, etc... and how quickly this happens is what defines the speed of sound in that material.

Well, on the other side of the object, the atoms get knocked back, but there aren't any other atoms of the material there to bump into -- instead, they bump into the atoms of the air. And those get knocked back through space until they hit something, and so on and so forth.

Eventually, if one of those hits your eardrum, it stimulates some nerves, and you perceive it as sound.

And now you know what a sound wave is -- it's kinetic energy getting rapidly transferred from atom to atom through a series of collisions. (Well. Technically not collisions. At the subatomic scale, everything's a field force, so really it's the atoms repelling each other when they get too close together. But at the macro scale, that's a collision.)

Oh wow! That's really cool what a sound wave is! But since it's from atoms hitting all the way to the other side, does that mean when someone knocks on a door, the person inside the room hears it louder than the person outside (assuming that the person inside just happens to be right at the door)?

And by sound "in" an object you mean it's because the atoms are in the object?


From what I remember, friction happens because of temporary dipoles..and those cause a sticky effect between objects. Buut why does electrical resistance occur? From too many electrons bumping into each other o_o?

Well, maybe, maybe not. Acoustics is a very rich field, so I'm sure you can imagine that a lot of factors play into that perception.

So first off, being three-dimensional, that energy spreads out in a spherical wavefront. The farther from the source, the larger the sphere, so that energy is spread out over a greater area. This is, in fact, the primary reason why sounds get quieter the farther away you are. And since a sphere's surface is two-dimensional, you can observe that the loudness is inversely proportional to the square of the distance, that is, at twice the distance it's 4x quieter -- this is called the inverse square law and it comes up in a LOT of places. So if you're closer to the source of the sound than the other person, you might hear it louder.

Second, there's a loss of efficiency when the energy has to move between regions of different density, such as from a solid door into the gaseous air. This is because some of the energy gets reflected instead of transmitted -- just like light shining on a piece of glass; some bounces off instead of going through. This is why sound gets louder if you cup your hand around your ear (some of the sound bounces off of your skin instead of passing through, and it gets redirected into your ear) and why it gets muffled if there's a wall in the way. On the other hand, the sound might echo inside the room, allowing more of the energy to come to the other person's ear, but the sound on the outside will just disperse out into the air.

That effect also applies to the act of knocking itself. Some sound goes into the door; some sound reflects. Depending on how the door is constructed, it might transmit the sound well, or it might muffle it, and it might reflect the sound well (so you hear it better) or it might not (so it sounds a lot duller to you than on the inside).

There are other things, but I think that's enough of an answer for now. :P